Protamine sulfate, a strong polycation, is used extensively in clinical medicine to block anticoagulant activity to heparin, a strong polyanion. Recent studies indicate that protamine may be a major cause of morbidity and mortality in patients who undergo cardiopulmonary bypass and that these reactions are not predictable in most cases. Furthermore, new low molecular weight heparins may not be as easy to neutralize as commercial heparin. Therefore, a new agent is urgently needed, to replace protamine, to block the anticoagulant activity of heparin without itself having anticoagulant or procoagulant activity or capacity to cause Type I hypersensitivity or to act on complement. One major mechanism by which polyions exert activity in vivo appears to be through the complement system. These studies will investigate a variety of polyions for capacity to regulate complement using state-of- the-art assays (Section D.2). Polyanions (polydisperse heparin and monodisperse heparin oligosaccharides) will be examined in detail to determine the structure and activity relationship for activity on complement (Section D.3). Polycations will be studied to determine specific mechanisms by which these basic substances regulate complement and again to determine the structure and activity relationship (Section D.4). The mechanism by which polycations preferentially inhibit the classical and polyanions preferentially inhibit the alternative pathways of complement will be determined (Section D.5). Affinity chromatography will be combined with conventional chromatography to purify heparin oligosaccharides with high capacity to bind complement (Section D.6). These heparin oligosaccharides will be studies for binding affinity to protamine, complement and a variety of polycations (Section D.7). These investigations will also determine the sequences in Hep which have high affinity for C3 and the sequence in C3 to which Hep binds. These studies should provide a more complete understanding of the complex role that polyions play in regulating complement activity and could lead to the development of new pharmacological agents.